Corrosion inhibitors and method of using same



United States Patent 2,877,188 1 CORROSION INHIBITORS AND METHOD OF USING SAME Robert W. Liddell', Bethel, Pa, assignor, by mesne assignmerits; to Hagan Chemicals & Controls, Inc., a.corporationof Pennsylvania No Drawing ApplicationJuly27 1956 Serial No. 6005368 12 Claims. (Cl. 252-389) This invention relates in general toinhibiting. the corrosion of ferrous metals by aqueous solutions where the metals and/or their alloys are in contact with water or aqueous solutions under circumstances where nonferrous metals may also be present and in contactwith the aqueous media. v

It relates in particular to a method of drastically reducing corrosion of ferrous metals in closed heating and/or cooling systems, which are usually fabricated froma multiplicity of metals and metal alloys.

The abovementioned systems have come into widespread use in modern structures, particularly in' ofiice buildings, apartment buildings; and the likejwherea single closed system transporting a liquid, generally Water or aqueous solutions, often serves the dual" purpose of heating the building in cold Weather and cooling it in warm weather.

Although temperatures and pressures within the closed system will'vary depending upon the seasons of the year as well as upon operating conditions, these systems generally' operate in a temperature range of from 50 to 180 degrees Fahrenheit although sometimes they may operate at temperatures above the atmospheric boiling point and at a pressure of 50 pounds gauge or even higher.

Water used in these systems whether at" high, low, or intermediate temperatures generally tends to be quite corrosive to the system itself which may be constructed from as many as six or more different metals or metal alloys: For example, a single system may contain carbon steel, which of course is lower in' cost than most other structural metals, aluminum, brass, copper, 'tin, lead (the latter two metals being found in soldered joints) as well as alloys of these well known metals. Although there are a number of reasonably satisfactory specific inhibitors to protect steel from corrosion when that metal is the only one to be considered, I have not found any of the usual inhibitors entirely satisfactory for the protection of carbon steel and its alloys in the presence of certain nonferrous metals, particularly copper and its alloys, under the conditions encountered in closed heating and/or cooling systems.

. Although the mechanism is somewhat obscure, it has been known for many years that in closed systems of the type herein mentioned, the alkali metal nitrites, either potassium or sodium, particularly the latter, exhibit an inhibitive effect in the protection of carbon steel against corrosion, particularly if' the pH value or alkalinity of the aqueous solutions within the system are maintained at a figure of 7 or higher. However, the alkali metal nitrites leave a great deal to be desired when used in 2 Y preventing corrosion of. steel which is in contact with water that is also in contact with. copper, especially if the pH value falls below 7 or if the concentration. of nitrite in solution fallsv below 200 parts per million. A suitable additive has long been sought by workers skilled in the corrosion art but there are operational andv functional problems associated'with all of the additives which have been used so that the only materialaccepted for prevention of corrosion. of steel in systems where oxygen is present and under conditions. of low blowdown is chromate. This material has marked staining; power when leaks occur and certain persons are also susceptible to the dermatitis it causes.

I have investigated a number of compounds whichin theory should be effective in reducing the corrosivity of aqueous solutions toward steel since they reduce the attack on copper and thus cut down on' the possibility of copper steel'. couple formation in which the attack on the steel should be" greatlyincreas'ed; Such-materials as potassium thiocyanate, 3-aminothiotriazole, and S-trithiane actually increased the corrosivity of the sodium nitrite solutions toward steel under the same conditions.

Additionally, I have" investigatedsome of the well known inhibitors" which are" extremelyeffective in prot'ecting' copper and copper alloys against corrosion including the" following thiols: Z mercaptothiazole', 2'- mercaptobe'nzoxaz'ole, 2 'mer'captobenzimidazole, and 2- mercaptobenzothiazole. These organic compounds are used quite successfully in protecting copper against corrosion but my experience has heen that where 'sodium nitrite. is employed in an alkaline environment as in closed systems, even though the thiols show protection of copper against corrosion, they have a negligible effect in protecting'steelwhe'n compared tothe compound I- use.

I have discovered that small concentrations of the organic compound known as henzotriazole having the structural formula exert. a hitherto unobtainable inhibitory effect on. steel when used in conjunction with alkali metal nitrites in an alkaline environment in: therpresence 0ft copper or copper alloys. A. synergistic effect seems to result from com:- bining: the nitrite,.alkaline compound, and benzotriazole, since a higher degree of inhibition is obtained than the expected additive effect of the component parts of the mixture. Benzotriazole itself: exhibits no" inhibition of steel. corrosion in aqueous solutions even in an alkaline environment.

- Toievaluate: various inhibitors for-.this particular application, I prepared 500ml. samples of Pittsburgh: tap water in beakersto'which I added 1000 parts per million of inhibiton 1"X 2" cold rolled lowca-rbon steel. test specimens were carefully wei'ghed-and 'immersed in each beaker. Four 1"x"2 copper strips were alsoplaced in the beaker's; The solutions were continuously ainagitated 'at room temperature for 30" days. Makeup was approximately distilled water" andv25% tap water. At the end of the test, the specimens were removed, dried, and weighed.

' specimens 'bouates, silicates, and

'borax. I Borax I I It will'be notedlthat while copperloisses were negligible the protection afforded the steel" compounds such as i has an'added germicidalivalue objectionable slime growths at a minimum in certain I prefer to total weight of inhibitor but I have used concentrations from as little as 0.1% to as much as considerable protection of the steel surfaces. In systems having considerable exposed areas of copper compared to the volume of solution, the higher concentrations are desirable.

Concentrations of inhibitor in aqueous systems may vary between 500 parts per million to upwards of 3500 parts per million. Experience generally indicates the most economical yet effective concentration of total inhibitor to be used, ranges between 1000 to 2000 parts per million.

Within the mixture itself I may vary the ratio of nitrite to alkaline salt between 9 to 1 and l to 4, or in other words from about 90% nitrite-% alkaline salt to about 75% alkaline salt-25% nitrite.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. A corrosion inhibitor consisting essentially of (a) alkali metal nitrite (b) a mildly alkaline salt, and (c) benzotriazole, the ratio of (a) to (b) being from about 9 to 1 to about 1- to 4 and (c) being present in an amount which is from about 0.1 percent to about 2.0 percent of the total weight of the inhibitor mixture.

2. A corrosion inhibitor consisting essentially of (a) from about 90 percent by weight to about 23 percent by weight of an alkali-metal nitrite, (b) from about 9.9 per cent by weight to about 75 percent by weight of a mildly alkaline salt, and (c) from about 0.1 percent by weight to about 2.0 percent by weight of benzotriazole.

3. A corrosion inhibitor as described in claim 2 where the alkali-metal nitrite is sodium nitrite and the mildly alkaline salt is a compound selected from the group consisting of alkali-metal carbonates, bicarbonates, sesquicarbonates, silicates, phosphates, and borates.

2% or more with weight to about I I I Welght'Loss Per I I I y Day in Milligrams I Per Square No. I Inhibitor Declmeter Copper Steel 9 75 7 sodium nltrite I 'f'"" sodium tetraboratez. '2 2--..- Sergei as ,No. 1 with 1%benzotriazole 0.1 I 0.1

, a e 3 Same as No. 1 with 1% mercaptobenze 1 0:1 5,. 3

. thiazole added. I i

(The following testswere'run 501 14 days) i 4. .4 Same as No. 1 with 1% KSGN added.-- 0.2 a s9. 1 5 Same as No. 1 with 1% diBilhYldlthiO-' 0.2 0.6

carbamate added. I I y I (Thelollowlng tests were'rtmtor days) s Sangagblo.1wlth1%2thiazo1inethlone '0, 58.1 I y 'n e r 7 Same as No. l'with 1% 3-aminothiotri ,0 r 62.4 7

5201s added. I 8, SameasNo. 1with1%S-trlthianeadded- 0.1 50.2 9--... r. ,No inhlbitorusei. 2.3 I 80 the carbonates, phosphates, bicar-' the like can be. added in place of I I in e p g closed systems as well s affording some protection to I soldern, j r: I y y use'abo'ut 1% benzotriazole. based on the I I s phates,and 'borates', and

' about 2.0 percent by weight of benzotriazole. I

' (a) is about 74' percent by weight, 'by weight, and; (c) is about 1 percent byweight. I I I the corrosion of ferrous metalswhich are in contact with water which comprises adding to the waters. corrosion inhibitor consisting essenl percent alkali-metal nitrite, (b)

'tiallyof (a) about 90 (6) about I about 9.9 percenta mildly alkaline salt, and

'75 percent by weight lected from the group consisting of alkali metal carbonates, bicarbonates, sesquicarbonates, silicates, phos- (c) from about 0.1 percent to nitrite, (b) from about I '4. A corrosion inhibitor consisting essentially of (a) sodium nitrite, (b) sodium tetraborate, and (c) benzotriazole-the' amount of (a) and '(b) varying between about I (b) and about 25 percent amount of (c) being from 90 percent (a) and 10: percent (a) and 75 percent (b), the about 0.1 percent to about 2.0 percent of the inhibitor mixture.

5. A corrosion inhibitor consisting essentially of (a) from'about percent bylweight to about 23 percent by nitrite, (b) from about 9.9 percent by 75 percent by weight of sodium tetrawe'ight of sodium borate, and (c) from about 0.1' percent 7 by about'2.0 percent by weight of benzotriazole.

6.A corrosion inhibitor as (b) is about 25 percent 7. A methodof retarding 0.1 percentbenzotriazole.

weight to described in claim 5 where 8. A method of retarding the corrosion'of ferrous metals which are in contact with water which comprises adding to the water a corrosion inhibitor consisting essentially percent by weight to about 75 10. A method of retarding the corrosion of metals in contact with normally corrosive aqueous solutions which comprises adding to said solutions (a) from about 90 percent to about 23 percent by weight of sodium 9.9 percent to about 75 percent by weight of sodium tetraborate, and (c) from about 0.1 to about 2.0 percent by weight of benzotriazole.

11. A method of retarding the corrosion of ferrous metals in contact with normally corrosive aqueous solutions which comprises maintaining in said solution from about 500 to about 3500 parts per million of an inhibitor consisting of (a) from about 90 percent by weight to about 23 percent byweight ofsodiurn nitrite, (b) from about 9.9 percent by weight to about 75 percent by weight of sodium tetraborate, and (c) from about 0.1 percent by weight to about 2.0 percent by weight of benzotriazole.

12. A method of retarding the corrosion of ferrous metals in contact with normally corrosive aqueous solutions which comprises adding to said solution (a) about 74 percent by weight of sodium nitrite, (b) about 25 percent by weight of sodium tetraborate, and (0) about 1 percent by weight of benzotriazole.

References Cited in the file of this patent UNITED STATES PATENTS ferrous of (a) from about 90 percent to about 23 percent by weigh-tot an; alkali-metal nitrite, (b) from, about 9.9 I 7 percent by weight of a mildly alkaline salt, and (c) from about 0.1 percent by y weight to about 2.0 percent by weight of benzotriazole. I

' 9'. A'm'ethod of retarding the corrosion of ferrous l metals which are in contact with water which comprises adding to the waterv a corrosion inhibitorconsisting of I (a) from about 90 percent toabou't 23 percent by weight ofsodiurnnitrite, (b), from about 9.9 percent to about y of a mildly alkaline compound sei 

1. A CORROSION INHIBITOR CONSISTING ESSENTIALLY OF (A) ALKALI METAL NITRITE (B) A MILDLY ALKALINE SALTS, AND (C) BENZOTRIAZOLE, THE RATIO OF (A) TO (B) BEING FROM ABOUT 9 TO 1 TO ABOUT 1 TO 4 AND (C) BEING PRESENT IN AN AMOUNT WHICH IS FROM ABOUT 0.1 PERCENT TO ABOUT 2.0 PERCENT OF THE TOTAL WEIGHT OF THE INHIBITOR MIXTURE. 